1. Field of the Invention
The present invention relates to an ink jet printing apparatus that ejects ink from nozzles for printing, and more particularly to a printing apparatus of a so-called multipass system which uses a print head having a plurality of nozzle groups each consisting of a plurality of nozzles and performs a plurality of main scans over the same main scan print area of a predetermined print medium by using different nozzle groups to form different thinned out images during the main scans according to thinned out mask patterns associated with the respective scans to complete an image. More specifically, the present invention concerns a reduction in image degrading factors such as density variations and white stripes.
In addition to general printing apparatus, the present invention can be applied to copying machines, facsimiles with a communications system, word processors with a printing unit, and industrial printing apparatus combined with a variety of processing devices as well as to press-dying apparatus and etching processing apparatus.
2. Description of the Related Art
The printing apparatus, such as printers, copying machines and facsimiles, form an image made up of dot patterns on a print medium, such as paper and thin plastic plate, according to image information. These printing apparatus can be classified into an ink jet system, a wire dot system, a thermal system and a laser beam system by the printing method. A printing apparatus using the ink jet system (ink jet printing apparatus ) ejects ink (print liquid) droplets from orifices of nozzles of the print head onto a print medium to which the ink droplets adhere, thus forming an image.
With a large number of printing apparatus in use in recent years, a variety of demands are being placed on the printing apparatus, including fast printing, high resolution, high image quality and low noise. Of these, the ink jet printing apparatus can be chosen as being able to meet these requirements. The ink jet printing apparatus, because it ejects ink from the print head, must stabilize the direction of ink ejection and the amount of ink ejected to meet the above requirements.
Although the ink jet printing apparatus has made a variety of improvements on the printing apparatus body side, such as the provision of an ejection performance recovery device, the stability of the ink ejection amount largely depends on the performance of each print head. In other words, the ink ejection amount and the ink ejection direction are greatly influenced by small errors produced during the print head manufacturing process, which include variations in the shape of orifices of the print head, in the electrothermal transducers (ink ejection heater) that generate energy for ejecting ink, and in electromechanical transducers (piezoelectric elements). This in turn results in density variations in the image formed, degrading the image quality.
An example of this phenomenon is shown in FIGS. 36A, 36B and 36C.
In FIG. 36A, reference number 1101 denotes a multinozzle head which, for simplicity of explanation, has eight nozzles 1102 (each of which, unless otherwise noted, generally includes an orifice, a liquid path communicating with the orifice, and an element for generating energy used to eject ink). Denoted 1103 are ink droplets from the nozzles 1102 which should ideally be ejected in the same amounts and in the same directions. When ejected in this ideal conditions, the ink droplets land on the paper, as shown in FIG. 36A, to form dots of equal sizes which in turn form an overall uniform image with no density variations, as shown in FIG. 36C.
In reality, however, the nozzles each have characteristic variations and when the printing is done as described above, the sizes and directions of ink droplet 1103 ejected from the nozzles 1102 will vary among the nozzles, as shown in FIG. 37A, forming the dots as shown in FIG. 37B. The figure shows that blank portions appear periodically in the head main scan direction, that there are portions where dots overlap excessively, or that a white stripe is formed as shown at the center of this figure. A collection of dots printed in this condition has a density distribution in the direction of nozzle array as shown in FIG. 37C which, when viewed by human eye, is perceived as density variations.
To deal with the density variation, the following method has been proposed. This method will be explained by referring to FIGS. 38A, 38B and 38C and FIGS. 39A, 39B and 39C.
This method performs three scans (main print scans) by a print head 2001, as shown in FIGS. 38A, 38B and 38C and FIGS. 39A, 39B and 39C to complete the printing in the print area shown in FIG. 36B. A four-pixel unit scan area measuring in the vertical direction of the figures one-half of the eight pixels of the head is completed by two scans (two passes). The eight nozzles of the print head 2001 is divided into a group of upper four nozzles (upper nozzle group) and a group of lower four nozzles (lower nozzle group). The dots printed in one scan by each nozzle are produced by thinning out image data to one-half according to a predetermined image data arrangement. During the second scan the remaining half of dots are embedded in the previously formed thinned out image to complete the printing of the four-pixel unit scan area. This printing method is referred to as a multipass printing method. This method reduces by half the influences of the nozzle characteristics on the printed image even when the print head has nozzles with variations in the ink ejection amount and direction as shown in FIG. 37A. So, the printed image will be as shown in FIG. 38B, rendering black and white stripes such as those shown in FIG. 37B less noticeable. The density variations therefore are alleviated significantly from FIG. 38C to FIG. 38C.
However, even with this multipass printing method it has been confirmed that the density variations may fail to be eliminated at all depending on a print duty of each main scan and that in a half-tone printing additional density variations occur. To deal with this problem, Japanese Patent Application Laid-Open No. 7-52465 (1995) proposes to set the pitch of each print area variable by randomly setting the amount of paper feed during the multipass printing. This randomizes the period of striped density variations to make them less conspicuous, thereby realizing a high quality image formation.
Further, Japanese Patent Application Laid-Open No. 8-25693 (1996) discloses a printing technique whereby images formed by two successive scans of the print head are partly overlapped. That is, of image data printed by the first scan, data printed in an area that is overlapped by the next scan is masked with a random mask pattern. Further, of image data printed by the second scan, data printed in an area that overlaps the area of the previous scan is masked with an inverted pattern of the random mask pattern. The image data thus obtained is used for printing.
Today, a quality of printed images has achieved a significant improvement thanks to an ever-increasing image resolution and a continuing advance of color printing technology. A technique being proposed and implemented to make further improvements in the image resolution involves reducing the amount of ink ejected per dot. Another proposed technique to realize an image quality that would match a silver salt picture involves simultaneously using light-colored inks with reduced densities in addition to the four basic color inks (cyan, magenta, yellow and black). It is, however, feared that reducing the amount of ink ejected per dot may cause problems (deviation of dot landing positions and ink ejection instability).
For example, when an image is formed by a print head having 256 nozzles at a 1200-dpi pitch, each with an ejection amount of 4 pl, an undesired phenomenon occurs in which the ink droplets ejected from the nozzles at the ends of the print head land on positions greatly deviated from where they are intended to land (this phenomenon is referred to as an end nozzle dot deflection). FIG. 40 shows a state in which the landing positions of ink droplets are greatly deviated at a paper feed boundary. With the print head with a 1200-dpi nozzle pitch and an ink ejection amount of 4 pl, the landing positions of several dots at the beginning of the printing are not deviated, as shown. As the carriage accelerates, however, the dot position begins to deflect, by about 50 μm.
FIG. 41 schematically shows the tendency of ink ejection from the print head 1101 as seen from the carriage when the droplets ejected from the ends of the print head 1101 have already begun to deviate. It is known that several nozzles 1102 at the ends of the print head tend to eject ink droplets somewhat inwardly, as shown in the figure. This tendency becomes conspicuous when an image is formed with minute droplets as small as 4 pl. The deflection of the droplets results in the formation of what is visually perceived as a white stripe. It is therefore conventional practice to increase the number of passes to make the white stripe visually less noticeable.
The above conventional technologies, however, have the following points to be further improved.
That is, in the technique described in the Japanese Patent Application Laid-Open No. 7-52465 (1995), although the random setting of the feed of the print medium randomizes the frequency of occurrence of the white stripes, it is desired to be further improved in reducing the occurrence of the white stripes.
In the Japanese Patent Application Laid-Open No. 8-25693 (1996), because the image area on the print medium printed by one scan and the image area printed by the next scan partly overlap with each other, the occurrence of the striped density variations is alleviated. However, when the precision of the landing positions of the ink droplets from the end nozzles of the print head degrades substantially as shown in FIGS. 31 and 32, the landing position deviation is visually perceived as a white stripe.
Further, in the techniques described in the above official gazettes, because the paper feed is controlled at variable pitches, as opposed to a normal constant pitch, it is feared that the throughput may deteriorate.
The throughput is also lowered when the number of passes is increased to make the white stripes less conspicuous as described above. The reduction in throughput is among the factors standing in the way to a faster printing speed required of the printing apparatus of recent years.